Mattis had opposed a quick withdrawal from Syria, and his resignation letter stirred concern about national security even among the president's staunchest allies, like Republican Sen. Lindsey Graham. Mattis' replacement is a former defense contractor with no military experience.

The announcement that Mattis would leave the administration — followed by a string of other top departures — took Washington by surprise earlier this month. In his resignation letter, Mattis cited differences with the president, in what many interpreted as a stern rebuke from a quiet and reserved retired Marine general who avoided the spotlight.

"You have the right to have a secretary of defense whose views are better aligned with yours," Mattis wrote in his resignation letter.

Here is what Mattis wrote to the Pentagon in his farewell letter, obtained by CBS News correspondent David Martin: